Question

3. The genome of living organisms includes coding and non-coding regions, plus the genetic material of mitochondria and chloroplasts (when present). Describe the mechanism by which genes express their function. (b) Consider an instance in which two alleles of a gene differ by one single nucleotide. How could these two alleles lead to substantially different functional outputs?

Answer 1

Answer 3:

(a) Mechanism of gene expression:

Gene expression is the process by which genes are used as a code to make proteins for the cell. A gene is expressed in response to a signal, which can be generated within the cell or the signal may be external. Proteins are required to carry out life processes.

Gene expression mechanism can be represented using the Central Dogma as follows:

DNA + RNA → Protein

Gene expression involves two stages, transcription (DNA TO RNA) and translation (RNA TO PROTEINS).

Transcription:

The process by which DNA is used to make a RNA molecule is called transcription. It takes place in nucleus and involves the following steps :

• When there is a signal for gene expression, certain transcription factors (TFs) bind to the promoter region of the gene to be expressed.
• This activates the transcription apparatus and RNA polymerase, the main enzyme for transcription, begins synthesizing RNA molecule in the direction 5'-3' using 3'-5' DNA template. Upon encountering a termination signal, transcription is ended.
• The nascent RNA molecule is subject to various processing, like methyl capping of 5' end, addition of adenine residues at 3' tail (polyadenylation) and splicing of introns (non-coding sequence).
• The final RNA molecule produced as a result of processing is called mature RNA or mRNA. It contains exons (coding sequence).

The single stranded mRNA molecule then leaves nucleus and enters cytosol for the next stage of gene expression, i.e. translation.

Translation:

The process by which codons (sequence of three bases) of mRNA are used as a guide to add amino acid sequentially and form the desired protein is called translation. It involves the following steps :

• mRNA binds to ribosome, which is the site of translation.
• Codons present in mRNA are used to incorporate corresponding amino acid. The amino acid is carried by a tRNA molecule to the ribosomes.
• The first codon is AUG, and corresponding amino acid is methionine, which is a common feature of all proteins.
• Amino acids are joined together by peptide bonds. When various amino acids are joined in a chain, a polypeptide (protein) is formed.
• When a termination codon is encountered in mRNA, translation stops.
• Release factors allow the polypeptide to dissociate from the ribosomal machinery.
• The protein so formed may be subjected to further processing in ER and golgi before becoming fully functional.

Answer (b) :

Amino acids are coded by a sequence of three nucleotides which constitute a codon. A change in single nucleotide would mean a different codon, which may code for an entirely different amino acid.

The function of a protein is dependent on its 3D structure. This structure is ultimately dependent on the sequence of amino acids present in the polypeptide. Since, different amino acids possess different properties, thus a change of single nucleotide may result into a change in 3D structure of protein. A protein with altered structure serves different function.

Hence, two alleles that differ by a single nucleotide may lead to entirely different outputs.